The present invention relates generally to monitoring the conductivity of a medium. More particularly, the present invention is directed to a long-life conductivity sensor system and method for monitoring the conductivity of a medium, such as concrete, by embedding or immersing a sensor within the medium.
Long-term monitoring of conductivity is often important in mediums such as concrete, soil and fluids. Steel is widely used in reinforcing concrete in buildings, bridges and roads. The conductivity of concrete can be used as a measure of corrosion of the steel. The electrical conductivity of concrete is usually low; however if concrete is contaminated with salt, then the conductivity of concrete will increase. And saltxe2x80x94more specifically, the chloride in saltxe2x80x94can cause the reinforcing steel to corrode. The steel reinforcement bars embedded in the concrete inherently have anodes and cathodes on the same surface, and the concrete acts as an electrolyte. Together, the anodes, cathodes and the electrolyte behave much like a short-circuited battery. This electrochemical process causes corrosion of the anodic areas of the steel, leaving the cathodic areas intact. If the concrete has been contaminated with chloride, the conductivity of the concrete will increase, which will accentuate corrosion, and eventually destroy the steel.
The conductivity of soil is a useful parameter in agriculture because conductivity correlates generally to soil grain size and texture: sands have a low conductivity, and silts have a medium conductivity. Clay can have a high or low conductivity, depending upon its mineral content. Conductivity in shale and bedrock can vary with season due to flooding and water entrapment. Soils that have moderate conductivity, are medium textured, and have medium water-holding properties are often the most agriculturally productive. Furthermore, long-term soil conductivity data can be used to estimate the corrosion rates of buried metal pipelines and metal conduits. Such data can also provide an indication of leaks and plumes originating from storage tanks and waste management areas, and contamination/pollution in soil sub-surfaces.
Many scientific disciplines require accurate, long-term monitoring of the conductivity of fluids. Environmentalists generally use conductivity data to determine seasonal changes in the salinity of lakes and oceans. Chemical engineers are able to use conductivity data to monitor corrosion rates in large holding tanks and industrial equipment.
The conductivity data in all of the above examples are generally obtained using one of two types of conductivity sensors. One type is the inductive sensor, and the other is the electrode sensor. The inductive-type conductivity sensor uses a toroidal input transformer to induce a voltage in an electrolyte medium. A toroidal output transformer measures the induced current, which is a function of the conductivity of the medium. Because of the nature of transformers, this type of conductivity sensor is relatively large and is less common than the electrode-type conductivity sensor.
Electrode-type conductivity sensors are further divided into sensors with two electrodes and sensors with four electrodes. In the two-electrode sensors, generally, a current (I) is induced between the two electrodes by applying a potential or voltage difference (from an external power source) between those electrodes. The voltage difference (xcex94V) and the current are measured and recorded. The ratio of voltage difference to current (xcex94V/I) provides the resistance from which the conductivity (xcexa) is computed as follows:
xcexa=k(I/xcex94V)xe2x80x83xe2x80x83(Eq. 1)
where k is the cell constant with units of cmxe2x88x921 or mxe2x88x921, and xcexa is conductivity with units of ohmsxe2x88x921 cmxe2x88x921, ohmxe2x88x921 mxe2x88x921, mho cmxe2x88x921, mho mxe2x88x921, S cmxe2x88x921 or S mxe2x88x921. The value of k is determined for each pair of sensors using a medium of known conductivity.
The four-electrode conductivity sensors employ a second pair of electrodes. The first pair of electrodes passes a constant current between them and through the medium. The second pair of electrodes measures the voltage difference between two points in the medium through which current is passing. For example, a constant current is applied across two outer electrodes, and the conductivity of the medium surrounding the electrodes is then calculated using the values of the voltage drop across two inner electrodes, the applied current, and the cell constant.
Prior art electrode-type conductivity sensors are widely adopted for spot-checking conductivity values and for short-term, in-line, and in-situ measurements. Existing applications include various types of industrial and environmental monitoring. However, a disadvantage of the prior art conductivity sensors of both the inductive type and the electrode type is that they have not been capable of long-term, in-situ monitoring across a wide range of conductivity values.
A prior art corrosion sensor intended for long-term, in-situ measurement in reinforced concrete is described in U.S. Pat. No. 5,895,843, issued Apr. 20, 1999, to Taylor et al. (the ""843 patent). It measures changes in the resistance in a steel wire that is buried in the concrete but not connected to the rebar steel reinforcement. The wire is the sensor: corrosive agents entering the concrete corrode the wire, thinning the wire and changing its resistance. This is an indirect method to infer corrosion of the rebar. Rather than monitoring the conditions including high conductivity that will eventually result in rebar corrosion, the method described in the ""843 patent detects corrosion only after damage to the wire, and by inference, damage to the rebar has occurred. Remedial action to correct a corrosive environment is likely to be more effective if taken early. Such early action is possible only if an environment is monitored directly, rather than simply detecting after-the-fact the deleterious results.
Finally, the voltage applied across the electrodes of an electrode-type conductivity sensor can introduce errors in the conductivity measurement. The conductivity of a medium is based on ion mobility. When a DC voltage is applied across the electrodes of a conductivity sensor, the ions near the electrodes are quickly depleted and the electrodes become polarized. Such polarization results in measurements that are higher than the actual resistance between the electrodes. Techniques using AC voltage have been developed to overcome this problem (see, for example, U.S. Pat. No. 4,751,466, issued Jun. 14, 1988, to Colvin et al.); however, such techniques employ complex AC waveforms that require sophisticated electronic components, which add to the cost and size of the corresponding sensor systems.
Therefore, particularly in the above-described areas of steel-reinforced-concrete, soil, and fluid monitoring, a need exists for a long-life conductivity sensor that may be permanently installed in a location, that reliably monitors conductivity changes over several orders of magnitude over a period of months or years, and that provides early warning of potential corrosion.
The present invention, among other things, presents a solution to the previously discussed disadvantages associated with prior art conductivity sensors.
It is an object of the present invention to provide a conductivity sensor system that may be embedded in solids such as concrete or soil, or immersed in fluids such as chemical reagents found in holding tanks, to monitor changes in conductivity over several orders of magnitude over long periods of time, up to several years.
Another object of the present invention is to provide a conductivity sensor system that is compact in size.
Yet another object of the present invention is to provide a conductivity sensor system that is of relatively low cost such that numerous conductivity sensors may be used in a single project, for example, embedded in a reinforced concrete bridge, while not significantly increasing the overall cost of the project.
Yet another object of the present invention is to provide a conductivity sensor system that overcomes the electrode polarization problems associated with some prior art conductivity sensors.
These and other objects are achieved in the present invention in a conductivity sensor system having at least a housing with an enclosing wall that defines an interior volume and that has at least one aperture through the wall; a pair of electrodes in contact with a medium surrounding the sensor housing; and conductivity-sensing electronics contained within the sensor housing interior volume and connected through the aperture in the wall to the pair of electrodes. The conductivity-sensing electronics include a galvanostat connected to the electrodes for inducing discrete, constant current pulses between the electrodes, creating a transient voltage signal between the electrodes; and a high-speed voltmeter/A-D converter connected to the electrodes for measuring the transient voltage signal between the electrodes, the transient voltage signal being a function of the conductivity of the medium surrounding the sensor housing.
Other objects and advantages of the invention will become more fully apparent from the following, more detailed description and the appended drawings, which illustrate several embodiments of the invention.